1. Field of the Invention
The present invention relates generally to a method for purifying silanes such as tetraethylorthosilicate (TEOS) to remove boron impurities therefrom, as well to as a method of analysis of analyzing boron impurities in such silanes.
2. Description of the Related Art
In the fabrication of microelectronic devices, current thin-film fabrication techniques require ultra-high purity precursors for depositing specific films. As device densities increase and critical dimensions decrease, electrical requirements of constituent thin-films become increasingly critical.
For example, dielectric films used to insulate gate and multi-level interconnect structures are commonly formed of silicon dioxide (SiO2) and must possess specific electrical properties. To reliably and reproducibly achieve such electrical properties, the precursors used to deposit the corresponding silicon dioxide thin-films must be of ultra-high purity.
Such precursors include tetraethoxysilane or tetraethylorthosilicate (TEOS). A common impurity in TEOS is triethylborate (TEB). Boron can form oxides of differing electrical properties relative to SiO2. Boron is also a dopant species that is extensively used to influence the charge carrier properties of silicon. In ultra-thin gate dielectrics, the diffusion of boron impurities can change the dopant concentration in silicon and thus, alter the performance of a specific transistor junction.
For this reason, the presence of excess boron in the SiO2 layer can be problematic. In fact, it has been shown that boron impurities ranging from 40-80 parts per billion (ppb) can result in boron concentrations in the deposited film of 1017 atoms/cc as determined by SIMS (see R. K. Laxman, A. K. Hochberg and M. J. Jahl, xe2x80x9cBoron Determination in TEOS and Reduction of Boron in SIMOX-SOI Processxe2x80x9d).
For these reasons, integrated circuitry (IC) manufacturers require boron concentration to be  less than 1015 atoms/cc in the SiO2 film for optimized performance.
Since the film growth conversion efficiency for boron-containing molecules in TEOS is relatively high (R. K. Laxman, A. K. Hochberg and M. J. Jahl, xe2x80x9cBoron Determination in TEOS and Reduction of Boron in SIMOX-SOI Processxe2x80x9d), the concentration of boron impurities typically must be less than 10 ppb in the TEOS starting material to realize good device quality characteristics in the final product.
The present invention relates to removal and analysis of boron impurities present in silane materials, e.g., TEOS, in connection with the use of such silanes as chemical reagents.
In one aspect, the invention relates to a process for purification of a silane material containing a boron impurity, comprising contacting the silane material with a multifunctional chelating reactant (MCR) for reaction of the boron impurity therewith, to yield an organoborate chelate as a reaction product, and separating the organoborate chelate from the silane material to recover a purified silane material.
Another aspect of the invention relates to a method of determining the amount of boron impurity in a sample of a silane material containing same, comprising the steps of: contacting the silane material with an MCR for reaction of the boron impurity therewith, to yield an organoborate chelate as a reaction product; separating the organoborate chelate from the silane material to recover a purified silane material; and quantitatively assaying the amount of the organoborate chelate to identify the amount of boron impurity in the sample.
As used herein, the term xe2x80x9cmultifunctional chelating reactantxe2x80x9d or xe2x80x9cMCRxe2x80x9d refers to a chemical agent that is (1) reactive with boron species having the formula B(RRxe2x80x2Rxe2x80x3), wherein each of R is same or different and selected from the group consisting of hydroxyl, C1-C8 alkoxy and C1-C8 alkyl, and (2) reactive with such R, Rxe2x80x2 and Rxe2x80x3 groups to form protonated reaction by-products.
In one aspect of the invention the MCR is reactive with TEB to form a donor acceptor complex and alcohol as a reaction by-product.
Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.
Although described hereinafter primarily in reference to TEOS, it will be appreciated that the method of the invention is not thus limited, but rather is broadly applicable to the purification and analysis of other alkoxysilanes, as well as alkylsilanes, and mixtures of the foregoing. The ensuing discussion relating to TEOS should therefore be understood to encompass such other silane materials, as variant feedstocks to which the purification/analysis methodology of the invention is usefully applied.
The present invention provides a method for obtaining high levels of TEOS purity, suitable for semiconductor device manufacturing applications, in which the TEOS is useful to form SiO2 films of corresponding high purity.
The present invention also provides in one aspect a method of analyzing TEOS to determine the level of purity thereof, for applications such as integrated circuitry manufacturing requiring  greater than 99.999% and more preferably  greater than 99.99999 elemental purity of the TEOS reagent.
The present invention allows boron species having the formula B(RRxe2x80x2Rxe2x80x3), wherein each of R is same or different and selected from the group consisting of hydroxyl, C1-C8 alkoxy and C1-C8 alkyl, to be chelated and removed from the TEOS. Common boron impurities such as triethylborate (TEB) may be greatly reduced in the purified TEOS product, e.g., to levels  less than 10ppb boron.
The boron impurity that invariably is present in TEOS produced for microelectronics fabrication applications is triethylborate (TEB). The method of the present invention enables TEB to be reduced to levels  less than 0.001% and more preferably to  less than 0.00001 % in the purified TEOS product.
The purification and analysis reagent compositions useful in the practice of the invention may alternatively comprise, consist of, or consist essentially of any of the purification/analysis components hereinafter described, and such compositions may additionally, or alternatively, exclude or be substantially free of any components not specifically described herein as being included or includable in such compositions.
The purification/analysis compositions of the invention in one aspect comprise one or more multifunctional chelating reactants (MCRs) that react with boron to form an organoborate chelate, preferably a nitrogen-containing organoborate chelate, having a boiling point above that of TEOS, preferably at least 50xc2x0 C. above that of TEOS, more preferably at least 80xc2x0 C. and most preferably at least 100xc2x0 C. above such boiling point of TEOS.
The MCRs of the invention that are employed to form a nitrogen-containing organoborate chelate as the reaction product of the MCR and the boron impurity, can be of any suitable type. Such MCRs correspondingly include nitrogen, preferably as an amine functionality of the MCR, or alternatively in the form of imine or nitrile functionality. In the chelated reaction product of the MCR and the boron impurity, the electron-rich nitrogen constituent deriving from the MCR coordinates to the electron-deficient boron central atom to form a highly stable chelate as the coordination complex reaction product. A preferred class of such MCRs includes multifunctional organoamines, such as alkylamines and aminoalkylamines. A highly preferred MCR of such type is tris(2-aminoethyl)amine.
The MCRs of the invention in one embodiment include oxygen-containing functionality that reacts with the boron impurities in the TEOS to form the organoborate chelate reaction product. The oxygen functionality of the MCR is preferably a hydroxyl functionality, or alternatively carboxyl or carboxamide functionality. A preferred class of such MCRs includes multifunctional organooxy compounds, such as multifunctional alcohols, glycols and other polyols.
A particularly preferred class of MCRs in the practice of the invention includes multifunctional chelating species, e.g., organooxyamines, organohydroxyamines such as triethanolamine, trimethanolamine, and the like.
A highly preferred MCR is triethanolamine (TEA), which is reactive with TEB to form a donor acceptor complex including ethoxy groups, and reactive with the ethoxy groups of the borate to form ethanol as a reaction by-product. TEA serves as a boron getter even at low concentrations in TEOS, because the TEA nitrogen electron lone pair donates electron density to the boron vacancy to form a donor acceptor complex. The general structure of the TEB molecule is shown below in Formula 1: 
The TEA/TEB reaction is characterized by low energy and is kinetically rapid. Once the donor acceptor complex is formed, the three alcohol groups of the TEA react with the three-borate ethoxy groups to eliminate ethanol. The resulting (end product of the reaction) TEA borate is very stable and non-volatile in character.
TEA is a particularly preferred aminopolyalcohol MCR. TEA is however substantially insoluble in TEOS and therefore the TEA/TEB reaction is advantageously facilitated by vigorous stirring or boiling action in the reaction medium to ensure good contact of the reactants with one another. In some applications, it may be desirable to enhance the solubility of TEA in TEOS by use of TEA analogs, e.g., triolamines having alkane substituents such as butyl groups, pendant on the C1 carbons of the TEA molecular structure.
The MCR compositions of the invention are utilized in the purification/analysis method of the invention to react with the boron impurities of the raw or feedstock TEOS to yield an organoborate chelate which is readily removable from the bulk TEOS by simple distillation to produce TEOS that is purified of such boron impurity.
In the purification aspect of the invention, the resulting distilled TEOS is a high purity product suitable for microelectronic device manufacture, as a source reagent for silicon dioxide.
In the analysis aspect of the invention, the wide separation of the boiling points of the chelate and the TEOS or other silane material, permits the chelate to be quantitatively recovered and assayed to determine the amount of the boron impurity in the TEOS or other silane material with a high precision. After quantitatively removing and recovering boron, conventional analytical methods such as ICP/MS may be used for further analysis of the recovered material. The invention thereby enables ultra-high purity (ppb) analyses of TEOS, related alkoxysilanes, alkylsilanes and mixtures thereof, for semiconductor applications.
By virtue of its high boiling point in relation to TEOS, the organoborate chelate is readily removed by distillation from the TEOS with which the MCR has been contacted, to yield a high purity (99.999999%+ and preferably 99.9999999%+purity, on a weight basis) TEOS product.
The MCRs employed in the invention have boiling points that are sufficiently different from the boiling points of either TEOS (boiling point=168.9xc2x0 C.) or the organoborate chelates produced by the MCR reaction, so that unreacted MCR does not itself interfere with the distillative process in the purification/analysis method of the invention.
In a preferred embodiment of the method of the invention for purification of TEOS, the MCR composition is added to the TEOS feedstock (boron impurity-containing TEOS) prior to or during distillation. The reaction between the MCR and the boron impurity is thereby carried out at ambient temperature, or at elevated temperature, as necessary or desired in a given application of the invention. Elevated temperature conditions will favor the reaction kinetics of the organoborate formation reaction, and the purification process system for such purpose can advantageously include a reaction chamber upstream of the distillation apparatus in which the MCR composition is added to the boron impurity-containing TEOS. The reaction chamber in such arrangement can be heated to facilitate the reaction forming the organoborate chelate, and the reaction chamber is suitably sized to provide the appropriate contact/residence time for the reaction.